The present invention relates generally to a keyless entry system for operating vehicular devices, such as a door lock device, a window regulator, a steering lock device, a trunk lid opener and so forth, without using a conventional mechanical key, such as an ignition key. More specifically, the invention relates to a keyless entry system which controls the aforementioned vehicular devices through a radio signal transmitted from a pocketable transmitter.
In the recent years, there have been proposed and developed various systems conveniently operating the vehicular devices without using the conventional mechanical keys, such as ignition keys.
One approach to a convenient vehicular device operating system has been which disclosed in the U.S. Pat. No. 4,205,325, to Haygood et al. Haygood et al discloses a keyless entry system for an automotive vehicle permitting a plurality of operations to be achieved from outside of the vehicle by one who is knowledgeable of preset digital codes. Functions such as unlocking the vehicle doors, opening the trunk lid, opening windows, operating the sun-roof or programming the system with a user-preset digital access code can all be performed by proper sequential operation of a digital keyboard mounted on the outside of the vehicle.
This and other conventional keyless entry systems require the user to accurately input the preset code through the keyboard. Although such keyless entry systems have been well developed and considered useful for eliminating the need for mechanical keys, a serious problem may occur when the user of the vehicle forgets the preset code. If the user is outside of the vehicle and the vehicle door lock device is holding the door locked, the user cannot unlock the door lock until he remembers the preset code.
In order to resolve this defect in the prior art and allow convenient use of the keyless entry system, there has been proposed a new approach in which a pocket-portable wireless transmitter, of a size comparable to a credit card and thus capable of being carried in clothing pockets, is used to identify users authorized to operate vehicle devices. The wireless transmitter always becomes active in response to operation or depression of any one of several push buttons to operate a desired vehicle device. This means that whoever possesses the transmitter has full access to the vehicle and that whenever the transmitter is near enough to the vehicle, keyless entry is possible for any one at all. As a result, if the user should lock the transmitter in the vehicle and leave the vehicle, anyone would be able to unlock the door, turn on the starter motor and steal the vehicle. In addition, it would be highly likely for items stored in the trunk and/or glove box to be stolen when transmitter is left in the vehicle.
In such prior proposed pocket-portable wireless transmitter type keyless entry systems, it is necessary to provide means for conserving the power used in radio communication in order to prolong the life of the battery in the transmitter. However, a relatively low power communication system greatly limits the range of radio signal communications.